As described in U.S. Publication No. 2010/0279024 entitled: “Reprogrammable Parallel Nanomanufacturing” of Thomas Crawford, which is incorporated by reference herein, magnetic recording can be employed to generate nanoscale magnetic field patterns on the surface of magnetic media. By exposing the surface containing these nanoscale patterns to a colloidal fluid containing magnetic nanoparticles (ferrofluid), nanoscale patterns with macroscopic dimensions can be created which are both programmable and re-programmable.
However, in order to keep ferrofluids well-dispersed in the fluid such that the nanoparticles do not clump together, when synthesized, the nanoparticles are coated with a chemical surfactant (for example, citrate). The actual process by which the particles do not aggregate involves a finite electrostatic charge on the surfactant molecules themselves. Since all of the magnetic nanoparticles have this surfactant coating on their surface, either electrically positive or negative, the net effect is that if the particles approach one another, a repulsive electrostatic force keeps them from aggregating into larger assemblies.
The negatively charged nanoparticle surfactant exerts a force that tries to push the nanoparticles away from each other, while the magnetic force tries to pull them toward the surface. Once assembled, this Coulomb repulsive force opposes the magnetic force pulling the particles to the surface and to each other. While nanoparticles can be assembled in fluid into patterns based on the underlying magnetic field nanostructure, the Coulomb repulsion, together with strong currents within the suspension fluid and the fluid surface tension as it dries, can be sufficient to overcome the magnetic force holding the particles to the surface and pull the assembled particles away from the surface of the media, such that when the fluid is removed, so is the pattern.
As such, a need exists for improved methods of forming a nanoparticle assembly on a magnetic media.